(CNN) Experts are turning their focus in the fight against Covid-19 to vaccinating young Americans -- warning that even though they don't face a high chance of serious illness, they still risk long-term symptoms if they get coronavirus.
The United States has so far made significant progress in vaccinating adults and a new milestone was achieved Tuesday.
Half of the adult population in the US is fully vaccinated, according to data published by the US Centers for Disease Control and Prevention.
More than 164 million people -- 49.5% of the total US population -- has received at least one dose of Covid-19 vaccine, CDC data shows. More than 131 million people, or 39.5% of the total US population, are fully vaccinated, CDC data shows.
At least 25 states, plus Washington, DC, have now fully vaccinated at least half of their adult residents, data published Sunday by the CDC shows.
But many experts have identified young Americans as a critical age group for inoculation success -- the key to getting the pandemic under control in the country.
People 24 and younger are getting vaccine doses at much lower rates. Among those 12 to 15, only 1.4% have received at least one dose, while 1.7% of 16- to 17-year-olds have and 7.6% of 18- to 24-year-olds have, according to data from the CDC.
To reach the threshold of protection needed to limit the virus' spread, at least 70% to 85% of the US population will need to be immunized through vaccines or infection, health experts say. Not only could vaccinating children, teens and young adults help reach that percentage, but leaving them unvaccinated could give the virus a chance to spread, mutate and develop a strain resistant to existing vaccines.
While vaccinated people seem to be protected against current strains, "there may be future variants for which we are not so lucky," said Dr. Megan Ranney, an emergency physician and director of the Brown-Lifespan Center for Digital Health.
The stable of vaccines available to adolescents could expand soon. Moderna announced Tuesday that its two-dose Covid-19 vaccine is safe and appears to be effective in kids ages 12 to 17. It plans to submit trial results to the US Food and Drug Administration in early June, along with a request for authorization to use the vaccine among children that age, the company said.
Pfizer/BioNTech's two-dose coronavirus vaccine was authorized May 11 for children ages 12 to 15.
With nearly 40,000 new cases, the US saw the lowest number of weekly Covid-19 cases among children since early October, according to the American Academy of Pediatrics.
Children accounted for nearly 20% of the new cases reported late last week. More than 3.9 million children have tested positive since the start of the pandemic as of May 20.
Depending on the state, children made up between 6% and 19.5% of those who were tested for Covid-19 -- and 5.2% to 34.4% of children tested were positive for the coronavirus.
Children represented 1.3% to 3.2% of total reported hospitalizations for Covid-19, based on the information provided by 24 states and New York City. Only 0.1% to 1.9% of all cases of Covid-19 in children required hospitalization.
Nine states reported zero child deaths among the 43 states that provided data on Covid-19 mortality.
For young Americans who feel hesitant or even unmotivated to get vaccinated, Dr. Anthony Fauci warned that even mild Covid-19 illness can result in life-disrupting impacts.
"There's a syndrome that is referred to as long Covid, which means that you get a syndrome following the clearing of the virus where it could be for months," the director of the National Institute for Allergy and Infectious Diseases said Monday at a town hall, adding that possible persisting symptoms include profound fatigue, muscle aches, temperature dysregulation and an inability to focus.
About 1 in 5 people between ages 18 and 34 who are infected with Covid-19 reported lingering symptoms beyond two or three weeks, according to a study last year by the CDC.
And adolescents and children still deserve protection against their risk -- however small -- of contracting a serious illness, Fauci said.
National Institutes of Health Director Dr. Francis Collins said Tuesday that the pandemic has been a significant challenge for children's mental health.
"People sort of shrug and say, 'well, you know, kids are resilient,' but wait a minute, this is really an incredible unprecedented challenge to their life experience," Collins told the House Appropriations committee.
Earlier research has shown that even otherwise healthy children have had some developmental setbacks and delays during the pandemic.
Protection for school-aged adolescents has increasingly come into focus as officials look ahead to the new school year.
Although a vaccine for younger children likely won't be available by the time school starts in the fall, Fauci told the House Appropriations committee on Tuesday that it should be safe for children of all ages to go back to in-person class then.
Given the percentage of teachers vaccinated already now and into the fall, "there should be no reason not to get children of any age back to in-person school by the time we get to the fall term," Fauci added. Several NIH leaders appeared before the committee to discuss the agency's proposed budget.
A vaccine for younger children likely won't be available until late fall, early winter, Fauci said.
"I don't have children that young, but I would not be concerned when we get to the fall," Fauci said.
New York Gov. Andrew Cuomo said children should be back in school full-time in-person statewide in September. His statement came after New York City Mayor Bill de Blasio announced that there will be no remote option for the city's public schools in the fall.
Currently, only students 12 years old and older are eligible for vaccines, though studies are underway on vaccinating younger children.
When New York City does open its public schools on September 13, students will still have to wear face masks and adhere to social distancing guidelines, NYC Schools Chancellor Meisha Porter said Monday.
"We would never take any risks with our most important assets ... our children," Porter said, adding that every school will be equipped with social and emotional support resources for students.
In California, the Los Angeles Unified School District announced that school campuses will reopen for in-person learning five days a week in the fall. Superintendent Austin Beutner recognized that not all families in the district will be ready to send their students back by then.
"We expect the vast majority of students, teachers and staff to be at school every day, but recognize that we must provide the online opportunity for those who need it," he said.
Understanding how coronavirus developed could have an impact on officials' ability to respond to future pandemics, but debate around its origin has grown.
After a US intelligence report found that several researchers at China's Wuhan Institute of Virology fell ill in November 2019, a former commissioner of the US Food and Drug Administration said on Monday that there is "growing circumstantial evidence" that Covid-19 may have come from a lab.
Chinese Foreign Ministry spokesman Zhao Lijian on Monday rejected the report and accused the US of "hyping up the lab leak theory."
"Through field visits and in-depth visits in China, the experts unanimously agreed that the allegation of lab leaking is extremely unlikely," Zhao said.
"I think the challenge right now is that the side of the ledger that supports the thesis that this came from a zoonotic source, from an animal source, hasn't budged," Dr. Scott Gottlieb said on CNBC's "Squawk Box." "And the side of the ledger that suggests this could have come out of a lab has been continuing to grow,"
Whether the virus derived from an animal or a lab, it is important for health experts to know, CNN medical analyst Dr. Leana Wen said.
"We need to understand what the origin is, if there is in fact an intermediary animal between bats and humans we need to understand because there may be a reservoir of disease we should be looking for," Wen said. "If this is indeed a lab leak, we should also understand for the purposes of securing lab safety protocols."
Among the unanswered questions are how seriously the researchers fell ill, when they got sick and if the virus was already circulating before then, Wen said.
The Chinese government has not been transparent on this issue, she said, but it is important the global community get down to the bottom of it.
"This is not the last pandemic that we are going to see and understanding the origin of this will help us to prevent something like this from happening in the future," she said.